Fix dataset sidecar write race condition, duplicate app init, and missing status/priority persistence

[Copilot]

Running the app under test revealed three bugs in the dataset pipeline: a race condition causing random 404s, a duplicate FastAPI app instantiation, and status/priority fields being silently dropped on every save.

Race condition in sidecar writes

_PROFILE_EXECUTOR background threads rewrote sidecar files via Path.write_text(), which truncates before writing. A concurrent read between truncation and completion would see an empty file, causing parse_sidecar("") → id "untitled" → get_by_id returning None → HTTP 404. Failures were non-deterministic and affected different tests each run.

Fixed by adding _write_text_atomic() — writes to a temp file in the same directory then os.replace() (atomic on POSIX) — applied to all three sidecar write sites: save(), save_with_file(), and save_profile().

@staticmethod
def _write_text_atomic(path: Path, text: str, encoding: str = "utf-8") -> None:
    fd, tmp_name = tempfile.mkstemp(dir=path.parent, suffix=".tmp")
    try:
        with os.fdopen(fd, "w", encoding=encoding) as fh:
            fh.write(text)
        os.replace(tmp_name, path)
    except Exception:
        try:
            os.unlink(tmp_name)
        except OSError:
            pass
        raise

Duplicate app = create_app() in app.py

Module-level create_app() was called twice, discarding the first instance. Removed the duplicate.

DatasetMetadata missing status and priority

DatasetMetadata (the YAML serialization layer) omitted these fields, silently dropping them on every save(). A PATCH updating status/priority would appear successful but return stale values on the next GET. Added both fields to DatasetMetadata.from_dataset(), to_dataset(), to_dict(), and from_dict().